Joining of metals using a metal braze is referred to as brazing. A brazing material having a low melting temperature and which comprises Sn and Pb is referred to as solder. Solder has been used as a joining material from long in the past. Since solder has a low melting temperature and a high joining reliability, it is widely used for assembly of printed circuit boards and electronic parts in electronic equipment. In printed circuit boards and electronic parts for electronic equipment, their surfaces are oxidized by exposure to air during storage as inventory after they are manufactured. As a result, joining cannot take place with solder merely by heating solder and a printed circuit board or an electronic part together. In order to join solder to a printed circuit board, an auxiliary material referred to as a flux is used. A flux has a function to remove oxides from the surfaces of the solder and the printed circuit board thereby making the surface of the printed circuit board easily wetted by solder.
Fluxes are classified as resin-type fluxes comprising a water insoluble resin such as rosin or a synthetic resin and an activator added thereto, water soluble fluxes comprising a water soluble resin such as polyethylene glycol and an organic acid-type activator which are dissolved in water or an organic solvent, and inorganic fluxes comprising an inorganic substance such as hydrochloric acid or zinc chloride. With an inorganic flux and a water soluble flux, it is always necessary after soldering to wash away flux residue, while a resin-type flux can often be used without washing. Therefore, soldering of printed circuit boards is primarily carried out using a resin-type flux.
Methods used for assembly of printed circuit boards and electronic parts in electronic equipment include manual soldering in which a solder wire impregnated with flux is melted with a soldering iron to carry out soldering, flow soldering in which rod-shaped solder is melted in a solder bath and a printed circuit board which is coated with a post flux is immersed in the solder bath, and reflow soldering in which a printed circuit board which is printed with a solder paste comprising solder powder and a flux is heated in a reflow furnace. Among these methods, flow soldering is widely used because of its suitability for mass production and its low cost.
A post flux used in flow soldering comprises a resin such as rosin or a modified rosin and an activator such as an amine hydrohalide or an organic acid dissolved in an alcoholic solvent such as ethanol or isopropanol. Rosin is a mixture of terpenic organic acids such as abietic acid and d-pimaric acid and exerts an activating action on flux. In addition, the residue of rosin remaining after soldering is essentially non-corrosive, non-hygroscopic, and electrically insulating at room temperature, thereby making it possible to use the flux without washing.
However, in soldering under conditions of high temperature and high humidity such that moisture condensation takes place, if flux residue is not washed away, the copper foil on a printed circuit board may react with active substances such as halogens in the flux residue and with water, resulting in the formation of a green-colored corrosion product having the formula CuCO3—Cu(OH)2 and referred to as verdigris. Verdigris is poisonous, and it is nearly insoluble in water and alcohols. Verdigris was frequently formed when inorganic fluxes based on hydrochloric acid or zinc chloride were used before highly reliable resin-type fluxes came to be used. At present, highly active fluxes which cause verdigris to form are no longer used for assembly of printed circuit boards or electronic parts, but even with resin-type fluxes, verdigris is infrequently formed under conditions of high temperature and high humidity which produce moisture condensation.
In addition to verdigris which is produced by corrosion, depending upon the flux, green-colored compounds are sometimes formed. In a resin-type post flux, an amine hydrohalide is used as an activator. If a large amount of an amine hydrohalide is employed in such a flux in order to improve the wettability of the flux, corrosive properties and insulating properties end up decreasing. Therefore, a large amount of an amine hydrohalide cannot be used. Instead, wettability is enhanced by adding an organic acid such as a dicarboxylic acid. However, if a large amount of an organic acid such as a dicarboxylic acid is used in a resin-type post flux, carboxyl groups of the dicarboxylic acid react with copper oxide and form a metallic soap of copper having a green color. This metallic soap cannot be distinguished by external appearance from verdigris, so it often becomes a problem during soldering of printed circuit boards.
It is possible to distinguish a metallic soap of copper from verdigris based on the fact that a metallic soap of copper dissolves in a solvent such as an alcohol, whereas verdigris does not. A metallic soap of copper which is formed due to using an organic acid is not corrosive and it does not cause a decrease in reliability, but it is easily mistaken for verdigris, which is indicative of corrosion. A post flux which prevents this problem by adding a gallate ester in order to inhibit a metallic soap of copper from forming has been proposed (JP 64-11094 A1).    Patent Document 1: JP 64-11094 A1